Hydrocarbon geller and method for making the same

ABSTRACT

An improved hydrocarbon gelling agent is disclosed comprising the reaction product of (1) a polyphosphate intermediate produced by reacting triethyl phosphate and phosphorous pentoxide and (2) a mixed alcohol having a substantial hexanol component, together with methods of making and using such gelling agent.

BACKGROUND OF THE INVENTION

This invention relates to an improvement in making gelled hydrocarbons,especially those used in hydraulic fracturing of subterranean formationsto enhance oil and gas production.

Hydraulic fracturing is a widely used method for stimulating oil and/orgas production. In performing a fracturing operation, a hydraulic fluid,usually a liquid hydrocarbon, is pumped into the well bore atsufficiently high pressure to fracture the surrounding rock formation toopen cracks in the formation through which oil and/or gas can flow intothe well bore. Since the cracks which are opened by the fracturing fluidtend to close once the pressure on the fluid is released, it iscustomary to inject into the well along with the fracturing fluid asuitable particulate proppant material such as sand. The small proppantparticles flow into the fractures created in the formation along withthe fracturing fluid and serve to prop the fractures open after thefluid pressure is released.

Proppant materials may be either lighter or heavier than the hydraulicfracturing fluid and thus may tend to float or settle out of the fluidprematurely, or otherwise be unevenly distributed in the fracturingfluid. To overcome this problem, it is customary practice to use gelledfracturing fluids which will hold the proppant material in suspension asthe fluid flows down the well bore and out into the formation fractures.This requires that the gelled fracturing fluid be of sufficientviscosity to hold the proppant material suspended in a slurry or matrix.At the same time, the fluid must not be so viscous that it cannot bereadily pumped into the well bore and out into the formation fractures.

Various materials are known which, when admixed with hydrocarbons, willcreate hydrocarbon gels of various viscosities. However, many of thesematerials are not suitable for use as hydrocarbon fracturing fluidsbecause of the particular requirements imposed by the environment inwhich they are used. The gels must be formed at or near the wellhead atambient temperature. Generally, several thousand gallons of normallyliquid hydrocarbon such as crude or refined oil, a gelling agent and anactivator are blended to form a gel. This mixture then is stored in fractanks until used. Due to various factors such as the amount offracturing fluid needed, labor schedules and other outside influences,the gel may be used promptly or a substantial period of time may elapsebetween the time the gel is initially formed and the time it is used.Therefore, it is desirable that the gel thicken relatively quickly to asufficient viscosity to suspend the proppant material, but at the sametime have stability over as long a period of time as possible so that itneither breaks down into a less viscous fluid, nor sets into a gel ofsuch high viscosity that it cannot be pumped into the formation, beforeit is used.

Additional complications are presented by the fact that, while the gelis formed at ambient temperature at the wellhead, it is utilized in deepformations where temperatures may be much higher. Depending upon thedepth, formation temperatures may be in the range of 200° to 250°Fahrenheit or higher. Such elevated temperatures tend to reduce theviscosity of the gelled liquid. Prior art gelled hydrocarbons which areof sufficient viscosity to suspend the proppant, but not too viscous topump, generally will not retain sufficient viscosity at temperatures of200° to 250° or more to retain the proppant in suspension.

An ideal gelling agent for forming a gelled hydrocarbon fracturing fluidwould be one which, when mixed with the normally liquid hydrocarbon andan activator, relatively quickly forms a gel of sufficient viscosity tohold the proppant in suspension and then retains its desired range ofviscosity for at least several hours at ambient temperatures. At thesame time, the gel should, when injected into downhole formations attemperatures of 200° to 250°, retain sufficient viscosity to keep theproppant in suspension for the period of time required for thefracturing operation, which may be up to eight hours. It is accordinglythe primary object of the rresent invention to provide an improvedgelling agent, hydrocarbon gel, and methods of making and using thesame, which meet these criteria.

DESCRIPTION OF THE PRIOR ART

Various gelling agents and methods are shown in the prior art forproducing hydrocarbon gels. See, for example,

the following U.S. Pats:

    ______________________________________                                               2,983,678     4,104,173                                                       2,983,679     4,152,289                                                       3,334,978     4,153,649                                                       3,338,935     4,174,283                                                       3,494,949     4,200,539                                                       3,505,374     4,200,540                                                       3,575,859     4,316,810                                                       3,757,864     4,537,500                                                       4,003,393                                                              ______________________________________                                    

None of these, however, discloses a hydrocarbon gel which will possesssufficient viscosity at ambient temperatures to retain the proppant insuspension, sufficient shelf life at ambient temperatures to permit thegel to be retained at the well site for at least several hours prior touse, and sufficient viscosity at elevated subterranean temperatures ofup to 250° to retain the proppant in suspension during the fracturingoperations.

Unable to meet these goals with a single gel, the prior art has utilizeda two-step method in which a first gel is formed which is stable atsurface temperatures and to which additional geller or activator isadded as the gel is pumped into the well to increase its viscosity atelevated downhole temperatures. See, for example, U.S. Pat. Nos.4,200,540 and 4,622,155 assigned to Halliburton Company. This two stepprocedure has significant drawbacks. Adding additional gelling agent oractivator "on the fly" as the gel is being pumped into the well makes itdifficult to assure proper measurement and mixing of the additionalingredients. Further, if the additional gelling agent or activator actstoo quickly, it can cause the viscosity of the gel to increase tooquickly, thus making it nonpumpable. If the additional agent acts tooslowly, the gel may reach the elevated temperature zone withoutdeveloping sufficient additional viscosity. The gel viscosity can thendrop, permitting the proppant to fall out of suspension. Obviously, itwould be far preferable to provide a single initial gel mixturepossessing the desired surface and the desired downhole viscosities atboth ambient surface temperature and elevated downhole temperature.

Nalco Chemical Company, assignee of the present application, previouslyhas produced a hydrocarbon geller under the designation "ASP-160." Thisproduct is an alkyl phosphate acid prepared by reacting approximatelythree moles of mixed aliphatic alcohols having carbon numbers 6 to 10and comprising approximately 4% C₆ alcohol, 53% C₈ alcohol and 43% C₁₀alcohol by weight per mole of phosphorous pentoxide to produce aphosphate intermediate and then reacting the intermediate withtriethylphosphate. The resulting alkyl phosphate ester is then blendedwith normally liquid hydrocarbon at the rate of approximately 6 gallonsper thousand gallons of hydrocarbon and crosslinked with a sodiumaluminate activator to produce a hydrocarbon gel. This gel exhibitssatisfactory characteristics at wellhead ambient temperatures, but tendsto lose viscosity rapidly at downhole temperatures exceeding 225°Fahrenheit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The improved hydrocarbon gelling agent or geller according to thepresent invention comprises the reaction product of (1) a polyphosphateintermediate produced by reacting triethyl phosphate and phosphorouspentoxide and (2) a mixed aliphatic alcohol in which hexanol is asubstantial constituent. Its ingredients are qualitatively the same asthose of ASP-160, but the proportion of hexanol used is substantiallyincreased, resulting unexpectedly in significant improvement in hightemperature viscosity of the gel, while maintaining a pumpable viscosityat ambient temperatures.

In producing the polyphosphate intermediate, it is preferred to use fromabout 1.0 to about 1.3 moles of triethyl phosphate for each mole ofphosphorous pentoxide, with 1.3 moles of triethyl phosphate being mostpreferred. Approximately three moles of mixed aliphatic alcohols foreach mole of phosphorous pentoxide used are then blended with thepolyphosphate intermediate.

The alcohol comprises a mixture of aliphatic alcohols having from 6 to10 carbons in their alkyl groups. Importantly, it has been discovered inaccordance with the present invention that by substantially increasingthe hexanol component of the mixed alcohol, the gelling agent is capableof producing a gel having a much higher viscosity at elevatedtemperatures in the range of 200° to 250° F., while retaining acceptableviscosity at ambient temperatures below 100° F. Accordingly, the mixedalcohol has an increased hexanol content of from about 13% to about 92%by weight of hexanol, with the remainder being divided principallybetween C₈ and C₁₀ alcohols. Most preferably, the mixed alcoholcomprises approximately 23% by weight of hexanol, 42% by weight ofoctanol and 35% by weight of decanol.

An improved hydrocarbon gel may be produced utilizing the gelling agentof the present invention by blending with a normally liquid hydrocarbona gelling amount of the gelling agent and an effective amount of analkaline aluminate activator. Typically normally liquid medium densitypetroleum fractions such as kerosene, gas oil, crude oil, or diesel areused. From about 6 to about 10 gallons of the gelling agent preferablyare utilized for each 1,000 gallons of hydrocarbon, with approximately 6gallons of gelling agent per 1,000 gallons of hydrocarbon being mostpreferred. Of the alkaline metal aluminate activators, sodium aluminateis preferred. Most preferably, an aqueous solution of sodium aluminateis utilized.

A gelled hydrocarbon produced in accordance with the present inventionwill set quickly at ambient temperatures in the range of 40° F. to 100°F. to produce a gel having a viscosity in the range of approximately 40to 80 centipoise, which is well within the pumpable viscosity range andis satisfactory for suspension of most or all normal proppants. The gelis stable in that viscosity range for 48 hours or longer. Further, thegel demonstrates a stable viscosity in the temperature range of 250° F.of approximately 100 to 200 centipoise, which is sufficient forretaining most or all well proppants in suspension.

When using the improved gelling agent to perform a well fracturingoperation, the gelling agent, liquid hydrocarbon and activator areblended in one or more frac tanks at or near the well site. The fractanks may be up to several thousand gallons each and from one to four ormore frac tanks may be utilized depending upon the size of thefracturing operation. Commonly, the frac tanks are filled in series sothat up to several hours may elapse between the time the hydrocarbon gelis formed in the first frac tank and the time it is formed in the lastfrac tank. Once the fracturing operation is begun, the gelledhydrocarbon is pumped from the frac tanks, mixed with the proppant andpumped down the well bore at the desired formation fracturing pressureand out into the formation fractures. The target formation may be keptunder elevated pressure for up to several hours to promote furtherpropagation of cracks. Therefore, it is desirable that the gelledhydrocarbon viscosity remain stable in the temperature range of 200 to300°F. for several hours.

Once the fracturing operation is complete, the pressure on the hydraulicfracturing fluid is released. However, gelled hydrocarbon trapped in thefractures might tend to prevent production flow from the fracturedformation back into the well bore. To avoid this, a neutralizing or"breaking" agent commonly is admixed with the gelled hydrocarbon as itis pumped into the well bore. The preferred breakers have a delayedneutralizing effect and thus tend to break down the hydrocarbon gel onlyafter several hours. One suitable such breaking agent is sodiumbicarbonate which may be admixed with the hydrocarbon gel in a finelygranulated or powder form. It has only low solubility in the gelledhydrocarbon and therefore its neutralizing effect is suitably timedelayed.

The following examples and experimental results will further illustratethe invention.

EXPERIMENTAL Funnel Times

In the following examples a funnel flow test was used to approximate thegelled viscosities at low shear rates, characteristic of a pumpable gel.In the procedure, a Baroid Marsh funnel is filled with 200 ml. of thesample gelled hydrocarbon. An electronic balance with a plasticcollection cup is placed below the funnel. A measurement is taken of thetime required to flow 100 grams of the gelled hydrocarbon into the cup,usually recorded as seconds per 100 grams. Generally a funnel time of200 seconds or less per 100 grams is considered a pumpable viscosity.

EXAMPLE 1

A 4-neck reaction flask was used, equipped with a thermometer,mechanical stirrer, condenser and gas (N₂) inlet. Triethyl phosphate(TEP) and phosphorous pentoxide (P₂ O₅) were mixed in a mole ratio of1.32 moles of TEP to 1.0 moles of P₂ O₅ and reacted for two hours at175° F. to produce a polyphosphate intermediate All additions andreactions throughout the experiment were conducted under a nitrogenblanket.

The reaction was exothermic and was complete after two hours at 175° F.The temperature then was allowed to drop back to 140° F. A mixture offatty C₆ to C₁₀ lineasr alcohols comprising approximately 23 weightpercent hexanol, 42 weight percent octanol and 35 weight percent decanolwas than added to the polyphosphate intermediate while holding thetemperature at 140° to 175° F. The fatty alochol mixture was addeddropwise to the polyphosphate intermediate. Three moles of the fattyalcohol mixture was used for each mole of phosphorous pentoxide used inproducing the intermediate. After completing the fatty alcohol addition,the temperature was allowed to increase to 250° F. to 260° F. and heldthere for two hours to complete the reaction. The reaction product wasthe geller in liquid form, an alkyl phosphate diester easily poured fromthe flask.

The liquid geller was readily soluble in a paraffinic hydrocarbon oil.The geller was mixed with kerosene in a Waring blender at a volume ratioequivalent to six gallons of geller per 1,000 gallons of kerosene (a "6gpt" ratio"). A sodium aluminate activator (comprising approximately 32weight percent sodium aluminate, 7 weight percent sodium hydroxide and61 weight percent water) was added dropwise in an effective amount toform the gelled hydrocarbon. It is believed that the gelling viscosityis imparted to the hydrocarbon by the formation of an alumino alkylphosphate ester as the reaction product between the sodium aluminateactivator and the alkyl phosphate diester.

After aging approximately twenty-four hours at 75° F., the resultinggelled hydrocarbon was found to have a funnel time of 128 seconds,representing a viscosity of 50 centipoise at 170 reciprocal secondsshear rate. At a temperature of 250° F., the gelled hydrocarbon had aviscosity of 140 centipoise and was stable at such viscosity range andtemperature for three hours.

EXAMPLE 2

The procedure of Example 1 was followed except that the geller tokerosene concentration was at a ratio equivalent to 10 gallons of gellerper thousand gallons of kerosene ("10 gpt").

The resulting gel after aging for twenty-four hours at 75° F. had afunnel time of 124 seconds and a viscosity of 78 centipoise measured at170 reciprocal seconds shear rate. When heated to 275° F., the gel had aviscosity of 179 centipoise. The viscosity declined 7% over a period ofthree hours at 275° F.

EXAMPLE 3

The procedure of example 1 was followed with the following changes:

(1) the mole ratio of triethylene phosphate to phosphorous pentoxide was1.0 to 1.0; and

(2) the mixed fatty alcohol comprised approximately 84 weight percenthexanol, 8.5 weight percent octanol and 7.5 weight percent decanol.

After standing for three days, the resulting gelled hydrocarbon had a 95second funnel time at 75° F., a viscosity of 45 centipoise at 75° F. anda viscosity of 195 centipoise at 250° F.

EXAMPLES 4 through 10

The procedure of Example 1 was repeated with the variations in hexanolcontent, geller to hydrocarbon concentration and TEP to P₂ O₅ ratioindicated in the following table:

    ______________________________________                                        Example  Hexanol Content Geller   TEP:P.sub.2 O.sub.5                         No.      (Moles)-(wt. %) Conc.    Ratio                                       ______________________________________                                        4        0.37-13.0       8gpt     1.32:1                                      5        0.75-23.0       8gpt     1.32:1                                      6        1.50-44.5       8gpt     1.32:1                                      7        1.50-44.5       8gpt     1.15:1                                      8        1.50-44.5       8gpt     1.0:1.0                                     9        2.62-84.3       8gpt     1.0:1.0                                     10       2.81-91.5       8gpt     1.0:1.0                                     ______________________________________                                    

The resulting gelled hydrocarbons had the following funnel times andviscosities:

    ______________________________________                                        Example     Funnel Time/                                                      No.         Viscosity (sec./cp.)*                                             ______________________________________                                        4           41/45                                                             5           39/42                                                             6           34/42                                                             7           238/60                                                            8           196/66                                                            9           143/60                                                            10          85/51                                                             ______________________________________                                         *75° F.; 170 reciprocal seconds shear rate; measured at 24 hours       aging time, except for example 10 which was measured at 2 hours.         

The foregoing disclosure and examples are illustrative only. As will beapparent to those skilled in the art, various changes may be made in theingredients, molar quantities, cure times, etc., within the scope of theappended claims, without departing from the spirit of the invention.

What is claimed is:
 1. A hydrocarbon gelling agent comprising thereaction product of (1) a polyphosphate intermediate produced byreacting triethyl phosphate and phosphorous pentoxide and (2) a mixedalcohol comprising from about 13% to about 92% by weight hexanol.
 2. Ahydrocarbon gelling agent according to claim 1 wherein said mixedalcohol comprises a mixture of alkyl alcohols having 6 to 10 carbons intheir alkyl groups.
 3. The hydrocarbon gelling agent according to claim1 wherein from about 1.0 to about 1.3 moles of triethyl phosphate areprovided for each mole of phosphorous pentoxide.
 4. The hydrocarbongelling agent according to claim 1 wherein approximately 3.0 moles oftotal mixed alcohol are provided for each mole of phosphorous pentoxide.5. The hydrocarbon gelling agent according to claim 1 wherein said mixedalcohol comprises approximately 23% by weight hexanol.
 6. A hydrocarbongelling agent comprising the reaction product of (1) a polyphosphateintermediate produced by reacting approximately 1.3 moles of triethylphosphate and approximately 1.0 moles of phosphorous pentoxide and (2)approximately 3.0 moles of a mixed alkyl alcohol, said alcoholcomprising approximately 23% by weight hexanol.